In the semiconductor industry, it is necessary to subject a wafer to a rethinning process after its structuring in order to reduce the axial or vertical dimensions and to realize electrical and thermal improvements of the finished product. In the rethinning process, the non-structured side of the wafer is removed by means of mechanical grinding methods. The electrically active layer and, if applicable, any contacting terminals provided on the wafer surface that are usually realized in the form of small soldering globules (pumps) remain untouched.
Since the rethinning process is usually realized by grinding the wafer mechanically as mentioned above, it is necessary to fix the wafer on a stable substrate in order to stabilize the wafer during the grinding process. This type of stabilization is particularly important for an already rethinned wafer because a wafer of this type only has a very slight mechanical stability.
In order to increase the mechanical stability of the wafer, it needs to be initially fixed on a substrate. However, the substrate needs to be removed again after the rethinning process is completed, namely without damaging the sensitive rethinned wafer during this process.
Numerous methods and devices are used in practical applications for producing this temporary connection between a wafer and a substrate. In all these methods, it is attempted to reuse the substrate for cost reasons. If silicone is used as the base material for the electric circuits, a silicon wafer is also used as the substrate in practical applications due to the favorable costs and the identical coefficient of thermal expansion. Substrates of glass, sapphire or other materials are used if the wafers consist of a different material, for example, GaAs.
One widely used method for fixing a wafer on a glass substrate consists of bonding the glass substrate to the wafer over a large surface. The adhesive used has the peculiar property of losing its adhesive properties once a certain temperature is exceeded. In order to separate the wafer from the glass substrate, energy is applied to the glass wafer by means of a laser such that the adhesive is dissolved. The wafer and the glass substrate can then be easily separated from one another. The disadvantage of this known method in practical applications can be seen in that the adhesive frequently is already destroyed by the temperatures occurring during the rethinning process such that the wafer already separates from its substrate during the rethinning process. This results in the destruction of the wafers.
Other known methods utilize films that are provided with an adhesive layer. This adhesive also loses its adhesive properties when a certain temperature is exceeded. Analogous to the previously described method, the high temperatures occurring during the rethinning process may already cause the connection between the substrate and the wafer to separate.
Adhesives that only lose their adhesive properties at temperatures higher than those occurring during the rethinning process cannot be used because the temperatures required for neutralizing the adhesive would destroy the structures of the wafer.